The present invention is directed to a bond structure for joining ceramic to metal surfaces, and in particular for bonding electrostatic chucks in semiconductor process chambers.
Ceramic surfaces are joined or attached to metal surfaces in a variety of industries for a variety of applications. For example, ceramic surfaces are joined to metal to form sealing joints between light bulbs and metal casings, joints between ceramic insulation and metal furnace skins, and various joints in semiconductor process chambers. In many of these applications, it is desirable to form a ceramic to metal bond that provides uniform and relatively high thermal transfer rates through the thickness of the joint interface. It is also desirable for the bond to withstand the thermal stresses arising from the large difference in coefficient of thermal expansion of the ceramic and metal materials. It is further desirable for the bond to be resistant to erosion or chemical degradation in the erosive gaseous environments, such as the gaseous plasmas.
Ceramic surfaces can be bonded to metal surfaces using commercially available polymeric adhesives. Although polymeric adhesives provide compliant bonds that can withstand high thermal stresses, the polymers typically degrade rapidly in erosive chemical environments such as oxygen plasmas, and have limited ability to withstand elevated temperatures. Also, the relatively low thermal conductivity of polymeric adhesives provides a bond having high thermal impedance and low thermal transfer rates through the thickness of the bond. In addition, polymeric adhesives are generally applied in relatively thick layers and variations in the thickness of the polymeric layer can result in variable thermal impedances across the bond layer.
In another type of bond or joint, the ceramic surface is bonded directly to the metal surface using a metal braze or solder. Such brazed bonds are generally less susceptible to chemical or erosion damage. However, the brazed bonds are subject to thermal stresses that arise from the large difference in thermal expansion coefficients between the ceramic and the metal braze. A large mismatch in thermal expansion coefficients can cause the bond to break or form microcracks at the bond interface. These microcracks eventually result in catastrophic failure of the bond and separation of the metal and ceramic material.
Yet another type of ceramic-to-metal bond uses a solid interposer layer having a thermal expansion coefficient that is half-way between the thermal expansion coefficients of the metal and ceramic surfaces. For example, the thermal expansion coefficient of copper of about 16 ppm/.degree. C., is approximately half-way between the thermal expansion coefficients of certain metal and ceramics, is commonly used for this purpose. However, a relatively thick copper plate is needed to withstand the bending stresses between the ceramic and metal layers without bowing of the layers, resulting in a thicker bond that provides reduced thermal expansion tolerance at high temperatures.
Ceramic to metal bonds that are used to join components in semiconductor fabrication apparatus have particularly high thermal conductance and erosion resistance requirements. For example, such bonds are needed to bond ceramic electrostatic chucks in the semiconductor process chamber. A typical electrostatic chuck comprises a ceramic insulator having an electrode embedded therein. The electrostatic chuck is bonded to a metal support or pedestal in the chamber. When a voltage is applied to the electrode, electrostatic attractive forces resulting from opposing electrostatic charges, hold a silicon substrate to the ceramic insulator of the chuck during processing of the substrate in the chamber. Because many processes can raise the temperature of the substrate to undesirable temperatures, the substrate and the electrostatic chuck are often cooled by conduction through the metal support. However, conventional ceramic to metal bonds have a low thermal impedance that provides reduced heat dissipation from the substrate through the chuck. These bonds also provide low resistance to erosion or failure in the process environment used to process the substrate.
Thus it is desirable to have a bond suitable for joining metal and ceramic layers that does not degrade or erode at elevated temperatures or in erosive process environments. It is further desirable for the metal to ceramic bond to have a low thermal impedance and a low variability in thermal impedance across the thickness of the bond layer. It is also desirable to obtain a compliant bond joint that can tolerate thermal expansion stresses without catastrophic failure. It further desirable to have a bond that provides high thermal transfer rates through the bond line.